This invention relates to speech signal processing and in particular to soft clipping circuits especially for an electronic circuit network in a telephone subscriber's instrument.
Conventional electromechanical telephone networks are inherently slow and, even under overload conditions, generate only minor harmonic outputs. In contrast the new electronic networks are fast and, when overload and consequent speech waveform clipping occurs, they produce an excessive proportion of harmonics that are outside the voice band (typically 300 Hz to 3.4 kHz). These harmonics are transmitted with little attenuation through the hybrid balance network, which is tuned to reject signals in the voice band, to the instrument receiver thus resulting in poor side-tone performance. Furthermore, in order for the hybrid to remain balanced and the return loss to remain greater than -14 db, as required by most telephone administrations, the impedance of the instrument as `seen` by the line should be 600 ohms (900 ohms in USA) irrespective of the signal amplitude. When electronic subsets at present in use are overloaded causing clipping the output transistor stage can be driven into saturation. When this occurs the impedance `seen` by the line falls to that of a saturated transistor, i.e. only a few ohms. Attempts have been made to prevent output transistor saturation by the use of clipping diodes. However, the impedance of the diodes then shunts the 600 ohm output impedance causing the aforementioned problems of impedance changes, hybrid imbalance and the generation of out of band harmonics.
The problem of harmonic generation may be overcome by the use of circuits which soft clip the speech waveform in contrast e.g. to diode circuits which produce hard clipping. A soft clipping circuit does not have a sharp cut-off level but progressively reduces the overall voice channel gain beyond a predetermined threshold amplitude. This provides a smooth transition from high to low gain thus preventing `splatter`. When soft clipping is applied to a signal substantially all the power is concentrated in the fundamental and the lower order harmonics, the latter being in or close to the voice frequency band. Thus, rejection to the receiver path by the hybrid network is still obtained.
Soft clipping alone, however, does not overcome the major problems of changes in the output impedance to the line and in particular the reduction of impedance caused by output stage saturation. The latter effect can be overcome by suitable choice of the circuit operating parameters which ensure that the waveform is always below the amplitude at which saturation occurs. The problem of impedance changes related to gain transformations has, however, proved intractable with conventional telephone circuits which employ negative feedback for gain regulation and, thus, suffer from an impedance reduction as the gain is reduced.